It isn't the pattern that is the problem. It is the lack of penetration.

I call it the column of lead fallacy. On one side of this fallacy people will argue that smaller shot size doesn't matter - that it's as effective as #00 Buck or a slug because since the shot hasn't patterned out - it acts like a slug.

On the other side of the fallacy people will argue that larger shot size doesn't matter, that since the #1 or #00 pellets haven't spread out enough to make separate holes, the shot is going to act like a slug, and won't have the same effect if the shot pattern had spread out and perforated the assailant multiple times.

Both of these arguments are wrong.

When a load of #1 Buck hits a target, even if the shot has not patterned out yet, the pellets will quickly begin to travel along their own paths - creating 16 distinct wound channels (although some of these wound channels may intersect at some points). The pellets will also penetrate to about the same depth as they would if they were more spread out - which can be tested by firing something like Federal FLITECONTROL® versus something that has no cup or even a spreader wad.

When a load of birdshot hits a target, even if the shot has not patterned out yet, the pellets will quickly lose velocity, penetrate to relatively shallow depths, and will also penetrate to about the same depth as they would if the shot were more spread out.

Pattern does not effect penetration.

Penetration is dependent on the mass of each pellet. A slug will penetrate the most deeply, #00 Buck will penetrate more deeply than #1 Buck which will penetrate more deeply that BB or any bird shot, and it's not dependent of the shot pattern.

This also gets confused with the argument that if you are firing a tight pattern and your shot placement is less than optimal - a tight pattern will be a non-debilitating miss, whereas a wide pattern gives you a better chance of at least one pellet disrupting vital tissue - but that's a separate argument.